1. Technical Field
The present invention relates to a piezoelectric oscillator, and particularly to a piezoelectric oscillator in which a driving current and a collector potential of an oscillation circuit are controlled based on an oscillation frequency of the piezoelectric oscillator and constants of a piezoelectric resonator.
2. Related Art
In recent years, a piezoelectric oscillator is used in many fields from communication device such as cellular phones to commercial-off-the-shelf devices such as a quartz-crystal clock due to its high frequency stability, compact size and lightweight, low cost, and the like. Among them, a temperature compensated piezoelectric oscillator (TCXO) compensating a frequency temperature characteristic of a piezoelectric resonator is widely used in cellular phones and the like that require frequency stability. JP-A-10-135741 discloses an oscillation circuit including a timer 51, a switch portion 52, a current supply portion 53, an oscillation portion 54, a changeover circuit 55, and a buffer portion 56 as shown in FIG. 6. The timer 51 measures an elapsed time from the time at which a power source potential is applied by turning on a power source, and outputs the measurement results when the elapsed time reaches a predetermined time. The switch portion 52 controls on/off between predetermined nodes based on a signal from the timer 51. The current supply portion 53 includes a current mirror circuit in which an output current flows through a constant current source. This current is set to a minimum value by which the oscillation portion 54 maintains the oscillation.
In the oscillation portion 54, one end of a piezoelectric resonator is connected to an output side of an inverter while the other end of the piezoelectric resonator is connected to an input side of the inverter. A feedback resister is connected in parallel with the piezoelectric resonator. The feedback resistor and the piezoelectric resonator form a positive feedback circuit. One end of a first capacitor is connected to the input side of the inverter and the other end of the first capacitor is grounded while one end of a second capacitor is connected to the output side of the inverter and the other end the second capacitor is grounded. Both capacitors stabilize the oscillation. The changeover circuit 55 is turned on when a signal S17 applied to a gate is “H”, so that a drain of an NMOS is grounded. As a result, the output of an oscillation signal to a subsequent stage is cut. In contrast, when the signal S17 is “L”, the NMOS is turned off. As a result, the oscillation signal is outputted to the subsequent stage. The buffer portion 56 has a function to shape a waveform and output an oscillation signal having a predetermined level.
JP-A-10-135741 discloses that, in the oscillation circuit structured as above, a starting-up time of oscillation can be shortened since the circuit is driven by a large power source current during a predetermined elapsed time from turning on the power source. In addition, the oscillation can be maintained with low power consumption since the oscillation circuit is driven by a small power source current after the predetermined elapsed time.
The oscillation circuit disclosed in JP-A-10-13574, however, does not solve problems facing a case in which a piezoelectric oscillator capable for covering a wide range of frequencies is structured by using a single IC circuit. Because the oscillation circuit is driven by a large current in an initial stage and is driven by a small current after the predetermined elapsed time in order to shorten the starting-up time of the oscillation. Demands for downsizing piezoelectric oscillators and reducing the costs have been increased drastically in recent years. To cope with the demands, oscillation circuits have been fabricated as integrated circuits. Fabricating an oscillation circuit as an IC requires high initial costs, leading to a problem in that costs per oscillator can be reduced by only mass production. To reduced the costs, a requirement arises that the IC is used in a wide range of frequencies, i.e., from 10 Mhz to 50 Mhz. This requirement requires that the piezoelectric resonator is downsized while increasing the frequency range, resulting in an effective resistance of the piezoelectric resonator being increased and frequency dips being produced. Improving the effective resistance and suppressing the frequency dips widen the values of constants such as the capacitance ratio of a piezoelectric resonator, and arise a problem in that a negative resistance does not satisfy a design value when an oscillator for a wide range of frequencies is made by using a single IC, or the oscillation is unstable or stopped by temperature change and the like even though the oscillator oscillates.